The present invention relates to an internal combustion engine system and a method for controlling the system.
A typical vehicle such as an automobile is provided with a fuel vapor processing unit, which traps fuel vapor produced in a fuel tank or the like with a canister and supplies the fuel vapor to an intake system with air using the intake air pressure (negative pressure) of the internal combustion engine so that the fuel vapor is burned in a combustion chamber. In such a fuel vapor processing unit, since most of the fuel vapor produced in the fuel tank is trapped by the canister, a large amount of the trapped fuel vapor must be supplied to the intake system of the internal combustion engine to prevent the canister from being saturated with the fuel vapor. The fuel injection amount in the internal combustion engine is corrected based on the flow rate (purge flow rate) of gas (purge gas) supplied to the intake system from the canister and the concentration (vapor concentration) of fuel included in the gas to suppress fluctuation of the air-fuel ratio in the internal combustion engine when the fuel vapor is supplied to the intake system of the internal combustion engine.
On the other hand, an exhaust gas recirculation (EGR) for returning part of exhaust gas to the intake system is performed in the internal combustion engine to improve the exhaust emission and the fuel consumption. However, to perform the EGR means to burn air-fuel mixture with the existence of gas that does not contribute to combustion in the combustion chamber. Therefore, if the ratio of exhaust gas (EGR rate) in the combustion chamber is excessively high during combustion, the combustion becomes unstable. Thus, the EGR rate of the internal combustion engine is controlled to be maximized within a range that does not excessively deteriorate the combustion based on the operating condition of the engine such as the engine speed and the engine load.
When supplying the fuel vapor in the canister to the intake system of the internal combustion engine with air while executing the EGR, the intake pressure in the internal combustion engine might approach the atmospheric pressure according to the execution of the EGR, which might decrease the purge flow rate. Therefore, Japanese Laid-Open Patent Publication No. 10-115258 discloses a technique for controlling the EGR rate such that execution of the EGR does not become an external disturbance to the purge flow rate so that the purge flow rate is maintained. The EGR rate in this case is also preferably maximized within a range such that the EGR rate does not become an external disturbance to the purge flow rate from the aspect of improving the exhaust emission and the fuel consumption.
A tank sealing system has been employed these days to suppress release of fuel vapor from the fuel tank. The tank sealing system is designed such that the fuel tank is hermetically closed in a normal state and releases gas in the tank to the canister when the pressure in the fuel tank is increased. In such a tank sealing system, the gas in the fuel tank is released to the canister when the engine is running and the fuel vapor in the canister is supplied to the intake system, that is, when the fuel vapor is purged. Accordingly, when the gas in the fuel tank is released to the canister, the fuel vapor contained in the gas is supplied to the intake system from the canister. This prevents the canister from being saturated with fuel vapor.
However, if the pressure in the fuel tank is increased and the gas in the tank is released to the canister when supplying the fuel vapor trapped in the canister to the intake system with air, that is, when the fuel vapor is purged, the gas flows into the intake system and causes the air-fuel ratio in the internal combustion engine to fluctuate. The reason for this is that, first of all, the concentration of the fuel vapor contained in the gas in the fuel tank is unclear. In addition, when the gas abruptly flows into the canister and the intake system as the gas is released from the fuel tank, the fluctuation of the air-fuel ratio caused by the introduction of gas cannot be sufficiently suppressed by correcting the fuel injection amount based on the purge flow rate as described above. When the air-fuel ratio fluctuates, the tendency of the combustion being unstable by the EGR becomes significant. Therefore, if the EGR rate is controlled to be maximized as described above when releasing gas from the fuel tank, the combustion becomes unstable due to the fluctuation of the air-fuel ratio caused by the release of gas from the fuel tank. Accordingly, deterioration of the driving performance might become unignorable.